Method of coating paper



July 4, 1 944 H. B. RICHMOND Erm. 21,352,658

METHOD OF COATING PAPER F-led Sept. 26, 1941 PIE-l1- @f @ff/2f' Patented July 4, 1944 2.352.658 y Marnon or comme rusa Howard Brewster Richmond and Gilbert K., Dickerman, Wisconsin Rapids, Wis., assignors to Consolidated Water Power & Paper Com.

Dany. Wisconsin of Wisconsin Rapids, Wis., a. corporation Application september zo, 1941, serial No. 412,362-

. y 12 Claims. This invention relates to improvements in a method of coating paper, paperboard or the like,

with=a mineral coating material by means of rolls to provide a coated surface upon the web which is ideally adapted for printing purposes, and refers specically to the control of the relative peripheral speeds of the coating roll or rolls and the translatory speed of the web to be coated and the relative peripheral speed of the rolls with `respect to each other in order that a smooth uniform film may be applied to the web surface.

The manufacturers of mineral coated papers and paperboard by roll processes have long encountered two serious production problems:

l. The occurrence on the coated surface of blemishes or irregularities resembling the pattern of a cats paw. These blemishes are sometimes referred to in the art as cat tracks.

2; The difculty in preparing a coated paper or paperboard in which both sides of the web have the same surface characteristics. The wire side of the web is usually rougher than the felt side and, hence, if both sides are coated with the been deemed necessary in order to solve the-prob lem. We have found, however; that a proper lcorrelation of the peripheral speed of-the rolls the correlation of the speed of the rolls with resame amount of coating material, the opposite faces of the web will have different characteristics. This problem is known in the art as twosidedness. The seriousness of this problem in the production of two-sided coated webs for books', magazines, cartons, and the like, is apparent. f

We have found that the nucleus of the cat track" is a small particle of solid coating material lodged on the surface of one or both of the coating applying rolls. As the surface of the web is' brought into contact with a coating roll containing such solid particle the coating is formed imperfectly around this nucleus. As the web leaves the coating roll a definite pattern is made at the point of this nucleus in the coated surface.

. We have found that cat tracks can be eliminated by the correlation of the peripheral speeds of the coating' rolls with respect yto the translatory speed of the web. That is, by driving the coating rolls at a peripheral speed greater or less' than the translatory speed of the web, the wiping action thus brought aboutY obliterates the cat tracks. n

The most expeditious way of curing the defect of "two-sidedness is to apply more coating material to the wire side-of the web than to the felt side thereof. Because of difficulties brought about by applying unequal amounts of coating material to the opposite sides of the web, expensive auxiliary coating apparatus has heretoforeand the translatory speed 4'of the webfsolvesthe problem of vtwo-sidetlness without the use of supplementary'equipment.'v

Another important factor in the production of a properly coated web'is the' draw," that is, the tension of the web in' passing through the-coat ing unit. If the coating roll or rolls are-run at the same peripheralspeed'asthe'speed'of the web, the draw Will'notfbe aiect'ed.- However,if the rolls are `run-a1; 'peripheral speeds 'different from the speed of the'web, and the speeds' are not properly correlated, the draw' will be dis'- turbed and the web lwill 'break or wrinkle, nega:- tiving a smoothly coated web.

Therefore; it is obvious-'that in eliminating cat tracks and curing'two-sidedness by varying the speed of the' rolls with' res'pect Ato the speed of the web, a'n'ew problem', the-draw, arises.-

Hence," our invention vis not only directed to spect to the web, but also' the correlation of the speed of the rolls with respect to'eachother.

The objects and advantages 'of lourinvention will be apparent from the accompanying drawing and following detailed'descrlption.

In the drawing,- Fig. 1 is a diagrammatic view of two juxtaposed coating' rolls'drivenl at equal peripheral speeds and'substantially'A equal to the translatory speed of the web'.

Fig. 2 is 'a similar view with the lower roll driven' at' aperipheral speed higher than that of the upper'rll and greater thanthe translatory speed oi!v the web.

Referring in" detail to the' drawing, I` indicates the upper. roll and 2 indicates the lower roll o'f a .pair -fnxtaposed foils-between which' a-web 3 passes.- vThe rolls l and' 2-may' have appliedto either, or both of their surfaces, by'means'of one or more contacting rolls (not shown) or 'otherwise, a mineral 'coating material. This type of coating unit is' well known tothe art, the web -3 passing between the rolls I and 2 being coated o one or both surfaces.

For purposesof description vboth rolls I and 2 will be considered as coating rolls and the web l. which may be paper or paperboard, willbe coated simultaneously onboth surfaces. Of course, our invention contemplates the coating of either one side or the other of the web as a separate operation or the coating of both sides simultaneously, as herein described.

pattern or cat trac results from imperfecta,

formation of the coating on the web caused by a particle of solid material lodged on thesurface oi' the coating rolls resulting in imperfect formation of coating on the rolls at this point.

This condition can be cured by rotating roll I at a peripheral speed greater orglesser than the translatory speed of the web. In so doing a wiping action is brought about which smooths sporters stance, the web and the lower coating roll, more coating 'material can be applied to the lower surface of the web while maintaining the web adhef:

ing consistently to the lower roll. Two-sidedness can therefore be cured and simultaneously cat trac which may tend to form are eliminated.

However, a thir'd factor must be considered and that is the draw. Generally speaking'the draw is the condition of tension in the web which moves it forwardly between two points in the paper machine. 'In passing the web between the coating rolls I and 2, the surface speed of said rollsbeing the same as the speed of the web, fthe draw will not be disturbed. However, if

" from the speed of the web and the other roll the coated surface and thus removes the-cat track formed. We have found the -web normally adheres to the surface which is moving at agi-eater peripheral speed. Hence, although the web may normally adhere lto the upper roll surface. it can be made to 'adhere to the lower roll surface by causing the lower roll to move at a peripheral speed greater than that of the upper roll. However, regardless'of whether the web adheres to one roll or the-other after leaving the nip, the differential in surface 'speed between said rolls and the web obliterates cat tracks."

Under normal lconditions the web consistently adheres, in passing between coating rolls to one 'or the other of the rolls for a longer distance beyond the nip. This is believed to be due to capillary attraction which may result from the roll to which the web adheres being smoother than the other roll, 'or one surface of the web being smoother than the-other, the smoother web surface tending to adhere to the adjacent coating roll `longer than the rougher surface adheres to its adjacent roll. In most instances it has been found that the web haga greater tendency to adhere tothe upper roll, and in the subsequent discussion of 'our invention this condition will be considered as obtaining.

Normally, the lower surface of the web 3 is one-of said rolls is driven at a speed diiferent maintained at web speed, the draw will be dis- "turbed and the web will tend to wrinkle or break.

Hence, in the situation where the speedv of the lower roll is different from the kspeed of the web to cure two-sidedness and eliminatev template varying the surface speed of each roll y with respect to the speed -of the web and also rougher than :the upper surface, and contains wire marks from the :forming wire. It is desirable to obscure'these marks by applying a greater weight of coating to the wire side of the web. Prior 'to our invention roll coater operators 'found it necessary to apply an equal amount of 'coating v`to each coating roll in order to assure the web would have a consistent travel through the coate'ing unit.' 'It was folmd `that the mere application of a greater amount of coating to the bottom roll than to the top rollwould cause the web, after passing through the nip of the coating rolls or through the nipof the coating roll and back-up roll (if a'sin'gle side coater is operated), to "ilop backand forth from top to bottom roll, adhering neither to the bottom nor to the top roll consistently. thusl causing vstreaks or mars on the coated surface. .Our invention in this respect consists of driving the lower roll (2) at a greater peripheral 'speed than 'the speed of the upper roll and in addition applying a greater quantity of coating to the bottom roll. These two factors i cause .the web to consistently adhere to the bottom' roll for a 'few inches after passing through the nip '(shown best at 5 in Fig. 2) thus accomplishing our purpose of correcting two-sidedness without additional coating equipment and still v with respect to each other.

instance roll 2, is increased say 5 percent, with respect to the speed of the web, a proper and satisfactory draw can be maintained by decreasing the surface speed of the roll I about y5 percent with respect to the speedof the web. This change in surface speed of the upper roll with respect to the web not only compensates for disturbance to the draw caused by the lower roll, but roll I,moving'"at a surface 'speed slower than the speed of the web tends .to obliterate cat tracks" which may otherwise be formed on the upper surface of the `web.

We have found that either roll may be driven at a surface speed between 25 and 10 per cent different from the translatory speed of the web. Preferably, however, a speed-differential of between .5 and 1.5 is deemed most satisfactory. By maintaining a speed differential'within the range specified. "cat tracks and two-sidedness can be cured without rendering the operation vtoo critical.

For example, the speed of the web 'l may'be 815 feet per minute; the peripheral speed of roll I may be 800 feet per minute and the 'peripheral speed of roll 2 may be 830 feet. In other words, rnll I may run 1.88 per cent slower than web I and roll 2 may run 1.88 per cent faster than web tlud roll 2 may run 3.76 per cent faster than ro A third factor enters into the problem, however. which must be observed and this factor l limits the relationship of the various speeds involved. This factor is the draw. It is preferable that for an ideal draw, the speed ofthe web be maintained about midway between the peripheral speeds of the rolls, that is, that the speed diiferential percentage `of one roll 'with respect to the web be substantially equal to the speed diiferential percentage of the other roll with respect to the speed of the web, said speed differentials being of opposite sign. However, the surface speed diilerential or percentage of difference in speed of one roll with respect to the speed of the web should not exceed by at most about 5 per cent and preferably not over 1.8 per cent the surface speed differential of the We have fourdl thatwhen the surface speed of one roll, foiI otherA roll with respectA to the web, the speed differentials being of opposite sign.

Within the limits stated herein, the following factors are of importance: The furnish of the web or sheet; the weight of the sheet; the absorptiveness of the sheet; the smoothness of the surface of the sheet; the amount of coating applied, and the viscosity and solids content of th coating material.

In general, the hereinbefore-mentioned factors influence the relative speed of the rolls and web and the relative speed of the rolls with respect to each other, as follows: With regard to the furnish of the sheet, a sulphite sheet is generally less absorptive than a groundwood 'sheet and normally the greater the percentage of groundwood in a sheet the greater is its absorptiveness. The more absorptive the sheet the faster the coating material is dehydrated when applied. 'Ihe side of the sheet on which the coating is most dehydrated, other factors being equal, lwill adhere to the coating roll after leaving the nip. Generally, therefore, to cause the absorptive sheet to adhere to a predetermined roll when it normally tends to adhere to the other roll, requires a greater speed differential than would be the case with a less absorptive sheet. In addition, a sheet having a partially ldehydrated coating has a more sticky surface characteristic than a less dehydrated coated surface, and, hence, a greater speed differential between roll and sheet is required to remove cat tracks.

Of course, in view of the fact that speed diierentials of the rolls with respect to the web and with respect to each other result in friction at the nip, greater speed differentials may be employed with stronger or heavier sheets, and lighter weight or weaker sheets dictate the use of lesser speed diierentials.

The absorptiveness of the sheet has hereinbefore been discussed with respect to the furnish of the sheet, and with respect to the smoothness of the sheet, generally stated, the smoothest side of the sheet tends to adhere to the roll. Normally, the smoothest side of the sheet is the most absorptive side thereof.

In view of the fact that the coating material tends to function as a lubricant at the nip and promote slippage, the greater the weight of coating, the greater speed differentials may be resorted to which is highly desirable; and vice versa, the lighter the weight of coating the lesser is passed between 'juxtaposed rolls rotating in opposit directions at least one of which carries a moble coating material upon its surface which is deposited upon a surface of the web, the improvement which comprises, driving each of the rolls at a surface speed o1' opposite sign of about .25 per cent to per cent diierent from the translatory speed ofthe web.

2. In a method of coating paper, paper board and the like felted fibrous webs wherein the web is passed between juxtaposed rolls rotating in opposite directions at least one of which carries y a mobile coat `g material upon its surface which roll at a surfacespeed of about .25 per cent to the speed differentials are which can be em- Below the lower limits set forth, no appreciable4 results are attainable in eliminating"cat tracks and obliterating wire marks, and above the upper limits specified all of the factors hereinbefore set forth, as well as others, become so critical that commercial operation is substantially impossible.

We claim as our invention:

1. In a method of coating paper, paper board and the like felted fibrous webs wherein the web 10 per cent faster and the other roll approximately the same percentage slower than the translatory speed of the web. l

3. In a method of coating paper, paper board and the like felted brous webs wherein the web i is passed between juxtaposed rolls rotating in opposite directions at least one of which carries a mobile coating material upon its surface which is deposited upon a surface of the web, the improvement which comprises, driving the coating roll at a surface speed of about .25 per cent to l0 per cent slower and the other roll approximately the same percentage faster than the translatory speed of the web.

4. In a method of coating paper,l paper board and the like felted brous webs wherein the web is continuously passed between juxtaposed ccating ro s rotating in oppositedirections and a mob' coating material is continuously deposited upon the surface of said coating rolls and transferred to opposed surfaces of said passing web, the improvement which comprises, driving each of said rolls at a surface speed of opposite sign of about .25 per cent to l0 per cent different from the translatory speed of the web to thereby substantially equalize tension on the web.

5, In a method of coating paper, paper board and the like felted fibrous webs wherein the web is continuously passed between juxtaposed coating rolls rotating in opposite directions and a mobile coating material is continuously deposited upon the surface of said coating rolls and transferred to opposed surfaces of said passing web, the improvement which comprises, driving one of said rolls at a surface speed of l about .25 per cent to l0 per cent different from the translatory speed of the web, and driving the other roll at a surface speed different from the translatory speed of the web, the percentage of difference in speed of each of said rolls with respect to said web speed being substantially equal and of opposite sign.

6. In a method of coating paper, paper board and the like felted fibrous webs wherein the web is continuously passed between juxtaposed rolls rotating in opposite directions and a mobile coating material is continuously deposit-ed upon the surface of one of said rolls and transferred to a surface of said passing web, the improvement which comprises, driving one of said rolls at a surface speed of about .25 per cent to 10 per cent different from the translatory speed of the web. and driving the other roll at a surface speed similarly different from the translatory speed of the web, the percentage of difference in speed of one of said rolls with respect to the web speed being not in excess of about 5 per cent of that of the other and of opposite sign.

7. In a method of coating paper, paper board and the like felted fibrous webs wherein the web is continuously passed between a pair of superimposed coating rolls rotating in opposite directions and a mobile coating material is continuously deposited upon the surface of said coating rolls and transferred to opposed surfaces of said passing web, the improvement which comprises, driving the lower roll at a, surface sped of about .25 per cent to 10 per cent faster than the surface speed of the web and about twice said per cent faster than the surface speed of the upper roll to eliminate cat tracks on the coated surface and obliterate wire marks on the web.

8. In a method of coating paper, paper board and the like felted fibrous webs wherein the web is passed between juxtaposed rolls rotating in opposite directions at least one of which carries a mobile coating material upon its surface which is deposited upor a surface ofthe web, the im,- provement which comprises, driving each of the rolls at a surface speed of opposite sign of about .5 per cent to 1.5 per cent different from the -translatory speed of the web.

9. In a method of coating paper, paper board and the like felted fibrous webs wherein the web is continuously passed between juxtaposed coating rolls rotating in opposite directions and a mobile coating material is continuously deposited upon the surface of said coating rolls and transferred to opposed surfaces of said passing web, the improvement which comprises, driving one of said rolls at a surface spee of about .5 per cent to 1.5 per cent different from the translatory speed of the web, and driving the other roll at a surface speed of about .5 per cent to 1.5 per cent different from the translatory speed of the web, the relative speed percentages of said rolls being of opposite sign.

10. In a method of coating paper, paper board and the like felted fibrous webs wherein the Web is continuously passed between superimposed coating rolls rotating in opposite directions and a mobile coating material is continuously deposited upon the surface of said coating rolls and transferred to opposed surfaces of said passing web, the improvement which comprises, driving the lower roll at a surface speed of about .5 per 'asuma cent toy 1.5 per cent faster than the translatory speed of the web, and about twice said per cent faster than the surface speed of the upper roll to eliminate cat tracks on the coated surface and obliterate wire marks on the lower surface of the web.

11. In a method of coating paper, paper board and the like felted fibrous Webs wherein the web is continuously passed between a pair of juxtaposed rolls rotating in opposite directions and a mobile coating material is continuously deposited upon the surface of one of said coating rolls and transferred to a surface of said web,.

the improvement which comprises, driving one of said rolls at asurface speed of about .25 per cent to 10 per cent different from the translatory speed of the web, and driving the other roll at a surface speed dierent from the translatory speed of the web, the speed difference percentage of one of said rolls with respect to said web being not greater than about 5 per cent different from the speed difference percentage of the other roll with respect to said web, the speed percentage differences between said rolls and said web being of opposite sign.

12. In a method of coating paper, paper board and the like felted fibrous webs wherein the web is continuously passed between a pair of juxtaposed rolls rotating in opposite directions and a mobile coating material is continuously deposited upon the surface of one of said coating rolls and transferred to a surface of said web. the improvement which comprises, driving one of said rolls at a surface speed of about .25 per cent to 10 per cent different from the translatory speed of the web, and driving the other roll at a surface speed different from the translatory speed of the web, the speed difference percentage of one of said rolls with respect to said web being not greater than about 1.8 per cent different from the speed difference percentage of the other roll with respect to said web, the speed percentage differences between said rolls and said web being of opposite sign.`

HOWARD BREWSTER. RICHMOND. GILBERT K. DICKERMAN. 

